There is an increasing demand that liquid crystal panels used in mobile phones, digital cameras etc. have proximity sensors in order to be made off when a user's face is positioned proximately to the liquid crystal panels, thereby reducing power consumption. Further, there is also a demand that the proximity sensors be used as range-finding sensors since output values of the proximity sensors are inversely proportional to detected distances. Further, there is also a demand that liquid crystal panels used in mobile phones, digital cameras etc. have illuminance sensors in order to control intensity of light from backlights of the liquid crystal panels in accordance with illuminance of disturbing light.
On the other hand, integrating-type analog-to-digital conversion circuits have a feature that they can realize high resolution with a simple configuration. This feature is desirable for devices which are slow in motion but are required to have high resolution (approximately 12-16 bits). Mounting both of an illuminance sensor and a proximity sensor on a device raises a problem that, for example, the device is required to have a larger size. Accordingly, there is an increasing demand that a single device is configured to detect both illuminance and proximity/non-proximity of an object.
FIG. 15 shows a first conventional example which is a general configuration of a sensor circuit 101 including an analog-to-digital conversion circuit. In the sensor circuit 101, an analog-to-digital conversion section ADC is an analog-to-digital conversion circuit which digitally converts the amount of current Iin outputted from a photodiode PD and outputs a digital value ADCOUT. The sensor circuit 101 includes a comparison circuit 102 (comparison section) for comparing the digital value ADCOUT which is the result of measurement with a threshold Data_th. The result of the comparison is outputted as a digital output signal Dout to the outside.
FIG. 16 shows a second conventional example which is a configuration of a photoelectric sensor to which the teaching of Patent Literature 1 was applied. The photoelectric sensor 103 includes a photodiode PD, an averaging circuit 104 (operation section), an offset value setting section 105, and a comparison circuit 106. The photodiode PD outputs a current Iin to an ADC (Analog-to-Digital Converter). The averaging circuit 104 generates moving average values of periodically obtained measurement values. The offset value setting section 105 makes addition or multiplication with respect to the moving average. The offset value setting section 105 adds an offset to the moving average or multiplies the moving average with an offset so as to obtain a threshold Data_th. The comparison circuit 106 compares the threshold Data_th with the digital value ADCOUT. The photoelectric sensor 103 can make accurate detection regardless of changes in external environmental conditions and changes derived from temperature change in a sensor.
FIG. 17 shows a third conventional example which is a configuration of an analog-to-digital conversion circuit 107 to which the teaching of Patent Literature 2 was applied. The analog-to-digital conversion circuit 107 includes a differential amplifier 108, a capacitor C, a constant current circuit 109, an oscillator 110, a counter 111, and a control circuit 112. The differential amplifier 108 constitutes a voltage follower, and an input voltage Vin is inputted to a non-inverting input terminal (+) of the differential amplifier 108. The capacitor C stores charges corresponding to the input voltage Vin. The constant current circuit 109 discharges the charges stored in the capacitor C. The counter 111 counts clock pulses from the oscillator 110 from beginning of dissipation of the charges until voltages at both ends of the capacitor C are constant. The control circuit 112 controls operations of the oscillator 110 and the counter 111. The analog-to-digital conversion circuit 107 can make analog-to-digital conversion of the input voltage Vin by use of a simple configuration.
Recently, proximity sensors are designed to include integrating-type analog-to-digital conversion circuits and circuits for driving light emitting diodes. FIG. 18 shows a configuration of a general proximity sensor to which the teaching of Patent Literature 3 was applied. The proximity sensor 113 includes a photodiode PD, a light emitting diode LED, and a control circuit 114. In the proximity sensor 113, the control circuit 114 drives the light emitting diode LED, the photodiode PD for light reception converts the light to a current, and the control circuit 114 determines whether a detection target 115 is positioned proximately to the proximity sensor 113 or not.
The proximity sensor 113 shown in FIG. 18 includes the sensor circuit 101 shown in FIG. 15. A sensor including the proximity sensor 113 is an illuminance sensor capable of determining whether measured illuminance exceeds a predetermined threshold Data_th or not.
FIG. 19(a) is a waveform chart showing a case where the detection target 115 is positioned proximately to the general proximity sensor 113 shown in FIG. 18. FIG. 19(b) is a waveform chart showing a case where the detection target 115 is not positioned proximately to the general proximity sensor 113 shown in FIG. 18. A difference between data Data1 during a period of driving the light emitting diode LED and data Data2 during a period of not driving the light emitting diode LED is referred to as difference data Data1−Data2. Broken lines in FIGS. 19(a) and 19(b) represent the full scale of the digital value ADCOUT. In the drawings below, when the digital value ADCOUT reaches the full scale, the digital value ADCOUT is saturated in the ADC of FIG. 15.
In a case where the detection target 115 exists, reflected light 116 resulting from reflection of light from the light emitting diode LED by the detection target 115 is intense. Consequently, the current from the photodiode PD is large, so that the difference data Data1−Data2 exceeds the threshold Data_th of the control circuit 114. As a result, a digital output signal Dout in a Hi level is outputted and the detection target 115 is properly determined (judged) as being positioned proximately to the proximity sensor 113 (FIG. 19(a)).
In a case where the detection target 115 does not exist, the reflected light 116 is weak, so that the current from the photodiode PD is small. Consequently, the difference data Data1−Data2 does not exceed the threshold Data_th of the control circuit 114. As a result, the digital output signal Dout in a Low level is outputted and the detection target 115 is properly determined as not being positioned proximately to the proximity sensor 13 (FIG. 19(b)).
Further, since a value measured by the proximity sensor is inversely proportional to the square of a detected distance, calculating the detected distance from the measured value enables the proximity sensor to serve as a range-finding sensor.